Transducers for acoustic flowmeter



April 5, 1960 .1. KRlTz 2,931,223

TRANSDUCEIRS FOR ACOUSTIC FLOWMEITER A T TOR/vf Y April 5, 1960 J. KRlTz 2,931,223

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TRANSDUCERS FOR ACOUSTIC FLOWMETER I Original Filed Deo. 10. 1954 5 Sheets-Sheet 5 d I 6e INVEMTOR. Jac/f /fr/fz gta-.1017 70%', @www/3W?? /4 T TORNE YS United States Patent TRANSDUCERS FOR ACOUSTIC FLOWNETER Jack Flushing, N.Y.,

Original application December 10, 1954, Serial No. 474,403. Divided and this application January 30, 1956, Serial No. 562,265

This invention relates to transducer devices and particularly to such devices for use in flowmeters for measuring the velocity characteristics of a fluid.

This application is a division of my copending application, Serial No. 474,403, filed December 10, 1954.

The present application is a continuation-in-part of my copending applications, Serial No. 67,503, 'iled December 27, 1948, now Patent No. 2,826,912, and'application Serial No. 209,295, led February 3, 1951 and Serial No. 209,296, filed February 3, 1951, now abandoned. Y *A By thev propagation of acoustic Waves traveling simultaneously in opposite directions in a fluid, it is possible to determine the flow velocity of the fluid independently of the velocity of propagation of acoustic waves therein. Similarly, it is possible to determine the velocity of propagation of acoustic waves in the uid, independently of any component of flow velocity of the iluid in the direction of measurement.

Transducers according to the invention, are particularly applicable to a owmeter having two loops for transmission in two different directions. Each loop consists of a transmitting and receiving means interconnected by a feedback circuit including an amplifier. and a wave packet generator. Each received wave packet is amplified and triggers the generator so as to cause wave packets tobe repropagated in a sustained manner. Any change in the iiow velocity of the uid will be accompanied by a corresponding change in the repetition frequency of the wave packets, and the magnitudes of the respective changes will be in constant direct ratio to the flow velocities being measured.

In a flowmeter having two feedback loops it is desirable to have the fluid paths of the loops equal in length inorder to obtain high accuracy. Where four separate transducers are placed in the fluid conduit, the equality of the paths is extremely diiiicult to obtain and hold. Moreover, when the two wave paths are physically remote from each other, even slight differences in the state of the fiuid in the different path regions cause slightly different acoustic propagation velocities resulting in large errors in ow measurement, especially at low vllow velocities.

One object of the present invention is to provide a transducer arrangement which results in substantially equal path lengths.

Another object of the invention is to provide a transducer arrangement in which theL wave paths in opposite directions through the uid are physically close.

Another object of the invention is to increase the amount of transmitted wave energy picked up by the receiving transducers.

A further object of the invention is to improve the shape of a pulse or packet of waves transmitted by the transducers.

I A further object `of the invention is rto improve the eticiency of supersonic frequency transducers.

2,931,22 ce v Patented Apr. 5,19 6 0 A further object of the invention is to` simplify the installation and improve the operation of the transducers of a flowmeter which utilizes bi-directional supersonic wave transmission through a fluid.

Other objects and advantages of the invention will be apparent from the following detailed description and the' accompanying drawing, in which:

Referring to the drawing:

Fig. 1 is a schematic circuit diagram illustrating an embodiment of the invention.

Figs. 2 and 3 illustrate the wave packets. v Fig. 4 is a partial sectional view of an improved transducer according to the invention.

Fig. 5 is a diagram of a wave packet of improved form.

Fig. 6 is a partial sectional view of another transducer according to the invention.

Figs. 7 and 8 are views of a dual transducer.

Fig. 9 illustrates an arrangement of dual transducers such as shown in Figs. 7 and 8.

Fig. 10 is an end view of another crystal arrangement view of a dual transducer Figs. 16 and 17 illustrate the operation of the transducers shown in Figs. 14 and 15.

In Fig. 1, an embodiment of the invention is illustrated utilizing four transducers mounted in a fluid conduit 10 for producing upstream and downstream transmission. The transmitting transducer 11 is arranged to transmit waves downstream to the receiver 12, while the transducer 11 transmits to the receiver12 in the upstream direction. The waves received by transducer 12 are impressed on an amplier 13, and the amplified waves are supplied to a wave packet generator 14, which already is described in my parent application Serial No. 474,403. When the amplified wave impressed on generator 14 reaches a predetermined amplitude, generator 14 is triggered and responds by producing a limited train of waves, or a wave packet. The wave packet is then impressed on transmitter 11, and after being propagated through the' thereby triggered again, and in this manner repeated wave.

packets are produced. The transducers 11?, 12', amplilier 13 and wave packet generator 14 similarly produce a continuous sequence of wave packets which travel through the uid in the upstream direction.

Mixer 15 has its two input terminals connected to the outputs of generators 14 and 14 respectively, and is provided with pulse signals whose repetition frequencies are f1 and f2, wave packets at the generator 14 and f2 is the repetition frequency of the wave packets at generator 14. The mixed signals are fed to a detector 16 which produces in its output, signals representing the difference or sum of the two frequencies. The mixer-detector combination is a form of heterodyne capable of producing an output vcurrent having components of a frequency equal to the sum of f1 and f2 and a frequency equal to the difference between f1 and f2. Either of these frequency components is selectedand fed to a frequency meter 17. The arithmetick difference )c1-f2 is directly proportional to the flow velocity of the fluid independently of acoustic waves therein. Similarly the arithmetic sum fyi-f2 is directly proportional to the propagation velocity of the waves inthe fluid ow of the fluid.

where f1 is the repetition frequency of thel converter or any other circuit the propagation velocity of the independently of the velocity of..

` 'Ihe triggering of the wave' packet generator 14 is explained with reference to Figs. 2, 3 and 5. Generator 14 produces a short packet of-high frequency oscillations at the resonant frequency of the transmitting and the receivingdransducers. The wavev form of such la packet is shown in Due to the'resonant character "of the' transducer and 'other vcircuit elements the signal received at generator 14 from amplifier 13 is not an exact replica ofthe voltage applid`to the transmitting transducer 11, but rather builds upy slowly in 4a fashion similar to that shown in Fig. 3. The wave packet generator 14 is normally quiescent and is triggered only when it receives a small predetermined voltage E. Consequently when the received voltage'has built up to this predetermined voltage, as indicated at P, the generator 14 fires; 4'

VWhile the. shape of the l,received wave packet and, particularly, the rate of signal buildup are functions of many variable elements, the period between successive z ero crossings of` the wave is a relatively stable quantity and is primarily determined by the inductive and capacitive values in the generator yand bythe' transducer characterstics.` In :any particular design, the instability ofthe ring time is therefore confined to that 1,/4 cycle region between the previous zero value of the wave and the maximum of the designed chosen triggering cycle. Thisy region is4 traversed with varying received levels due to uid variations or amplifier variations. Nevertheless, due to` the high frequency nature of the wave packet, the total possible time variation Within this Vrcycle is small. This therefore representsY a marked improvement in the time stability of the feedbackloop.

In a wave packet owmeter, the received vibrations build 11p t0 their maximum value at a rate determined by` the amount ofmechanical loadingA presented to the crystal transducers. It is desirable, however that the particularcycleofthe wave packet chosen for triggering the wave packet generator have, an amplitude considerably higher than the cycle immediately preceding it. This condition is desired in order to prevent tiring at therwrong cycle.` It is also desirable that-V the magnitude of succeeding` cycles not be considerably higher than `the chosen firing cycle so as to preclude interference in operation fromrandomreections.

It h as been foundz when using resonant piezoelectric crystals as transducers in direct contact with the lluid tobileasured, thatthe wave packet buildup time is, slow,` While its veventual maximum amplitude is high, as shown in Eig. 3. In usingsuch crystals, a. relatively early cycleV rnust housed; to take advantage of a relatively large dif.- ference in amplitude between successive cycles.

lAi transducer for producing a wave packet having an improved wave form is illustrated in Fig. 4. The piezoelectric crystal. 60, having a connection 64 is provided vigithva metal interfacel between it and the iiuid. Interface 61 may be provided by a cylindricalhousing 63, as shown in Fig. 4 or 6. The high acoustic impedance of suchl housing loads the crystal heavily and producesthe desired Ycharacteristicsdescribed above. The overall loss tothe early cycles ofvacrystal with such lan interfaceisV no hgherthan'whenthe crystal is in direct contact with the uid, duetov the increased power absorbed from the crystal by the high acoustic impedance loading. Infact,V

affxnaterial such as magnesium or aluminum may be chosen for` theA interface which gives a gain forithe early cycles overv thatvobtained by direct contact between the crystal` and the fluid. The crystal may 'beI mounted onY` the: interface 61 infany manner which assures an intimate Contact therebetween. One-method is to provide a pres-` sillfe mounting With a thin film ofcoupling uid between thefcrystal and-theinterface. Another is to provide a thin filtri of solidadhesive betweenthe surfaces. The housing 6,3 separates the crystalifrom the fluid;and has the irn- Pltanadvantageof providing chemical and mechanical protection. for thecrystal. The improvement given by thel metal; interfaceis shown qualitativelyY by'Fig. 5 -Yin comparison with Fig. 3. In Fig. 5, the second positive half cycle has a much greater amplitude than the first positive half cycle and thereafter the amplitude of the wave packet remains nearly constant. y

When the thickness of the interface 61 is greater than half the number of wavelengths of the wave packet, reflections from the metal-fluid boundary arrive back at the crystal when excitation` is no longer present, and the thickness no longer contributes to'the amplitude of the early cycle behavior. Thus thick` metal slabs can be used without altering the performance. In certain instances, however, it is desirable to control the thickness of inter: face 6,1v to a specified v alue lessr than half the number of wavelengths produced by the wave packet generator. For example, a plate having a thickness of a half wavelength will cause the secondv'cycleof the wave packet to be reinforced and give a further improvement, or conversely, a plate having a thickness of three quarters wavelength will tend to reduce, the amplitude after one and a half cycles thus reducing reflection effects.

In Fig. 6 a materialV 62 havingr an appropriate acoustic impedance is fastened to the metal interface 61. The material 62 has an acoustic impedance preferably lying, Abetween that of the fluid to be measured and that ofl metal housingl 63, in order to permit increased power transfer. When an additional layer such as. 62 is -desired for chem-` ical. protection of a more suitable crystal loading material, a layer having a very highv acoustic impedance can i be used if the layer `is extremely thin, or if itisA an integral number of half waves. thick but less than half the length of a wave packet. Y' v In previous. owmeters using two wave paths, crosse diagonal paths for acoustic transmission have been suggested, asfshown'in Figli. ASuche-n arrangement has; these, disadvantages: '(11)' great machining accuracy is ree. quiedto'` insure equal path lengths, and (2.) the paths.t are physically 'remote from eachother so that even slight; differences in the statel of theiuid yin the diiferent paths cause slightly different propagation. velocities whichy re, sult inlarge errors in flow measurement, especially, at low` flow velocities. My prior applications, of which this application is a continuationimpart, disclose hybrid and.V bridge methods which overcome the above noted objections. I shall now describe another meansof overcoming.

theseI objectionsl which approaches thek characteristics ofthe hybrid method but doesl not require accurately bal-v anced bridge orhybrid circuits.

In the transducer lassembly shown in Figs. 7V and 8;. the'met'allic cylindrical housing 63a has an end wall; 61a which may be dimensioned as described in connec, tion with Fig. 4, and may contact the uid directly or through an impedance matchingsection such as the mate. rial 62 -in lFig'. 6. Within the housing 63a and on. the; wall 611g a -pair ofV crystals 65, 66 are, mounted. One. of these crystals serves as av transmitter and the other;- serves as a receiver. VThe crystals are providedy with. leads. 67 and 68. The crystals may be semi-circular, although it will be understood that other crystal shapes can be used. Under certain conditions when thethickness of metal wall 61a. is` large, acoustic coupling between the crystals occurs. Such unwanted coupling is.` reduced` by a. diametrical notch 69 cut in the metal interfacela so as to separate the two active halves. of. interface 61a. An electrical shield 70 prevents electrical. coupling between the two crystals.

The dual transducer shown in Figs. 7k and 8 not only,` has the advantages of thetransducer shown in Fig. 4; but has. the further important advantage that it greatly simpliiesthe obtention of substantially equal and close.;

of the fluid conduit. The crystals 65 and 65' are transmitters and crystals 66 and 66 are receivers. The crystals in the two transducing devices are arranged to give two close and parallel paths 72, 73. Moreover, the lengths of the paths can be readily equalized with far greater accuracy than can be generally obtained by the use of four separate transducersarranged as shown in Fig. l for example. -To make the paths equal in length all that is necessary is to make the end walls of the transducer housings parallel.

Instead of using two separate crystals to form a dual transducer a single crystal slab 60a may be used. as shown in Fig. l0. On the slab 60a there are mounted two separate electrodes 75 and 76, which may be semicircular` having separate leads 67a and 68a.- It is to be understood that in any crystal configuration disclosed herein several crystals may be replaced by a single crystal having several electrodes.

Fig. l1 shows another dual transducer in which the housing 63h. crystals 65a and 66a, and leads 67a and 68a may be similar to those of Figs. 7 and 8. The crystals 65a and 66a are isolated or decoupled from each other by the groove 69a and electric shield 70a. The end wall 6lb has a curved outer face 77. The face 77 is curved to focus the transmitter waves to a line or a point by making the face 77 cylindrical or spherical. It will be understood that if the material of the housing 63b has an index of refraction greater than that of the fluid, the wall 6lb may require a convex rather than a concave curvature.

A pair of transducer assemblies of the type shown in Fig. ll are arrangedV in a pipe 71a to give a pair of very close crossed wave paths 8l, 82, as shown in Fig. 12. The crystal positions of one transducer assembly are represented by lines 65b, 66b, and of the other transducer assembly by lines 65e and 66e. The radius of curvature of the lensfsurface 77 for the desired paths 81, 82 is determined by the fundamental laws of wave propagation. considering the wave velocities in the metal and liquid media, and the deisred spacing between the transducer assemblies.

The arrangement shown in Fig. 12 has the further Y advantage that if. as shown in Fig. 1 3, the crystal seating surfaces 83 and 84 of the metal interfaces of a pair of .transducer assemblies are displaced from their parallel of wall 61e which is intermediate crystals 65d and 66d.

In other respects housing 63e may be similar to those described previously. The crystals 65d and 66d are provided with input and output leads 67b and 68b.

The operation of a pair of transducing devices of the type shown in Figs. 14 and 15 is schematically indicated in Figs. 16 and 17. In Fig. 16 the lens formed by wall 61e converges the waves produced by the transmitting crystal 66d into a beam 90 focused on the receiving crystal 65e of the opposite transducing device. Conversely the waves produced by the transmitting crystal 66e are converged into a beam 91 which is focused on the receiving crystal 65d. The beams 90 and 91 will be superimposed on each other. It is evident then that the two beams, which travel in the directions indicated by arrows 92, 93, will occupy nearly the same paths through the fluid, and that they will have substantially the same length by virtue of the fact that they extend between the same pair of transducing devices. yA further advantage of these transducing devices is that the waves are concentrated into a high intensity beam at the receiving crystals. The resulting system, therefore. has an increased discrimination against unwanted reections, and also requires less amplification for stable operation.

I have shown and described several embodiments of my invention and for the sake of simplicity have not illustrated every feature of my invention in connection with each embodiment thereof, but it will be evident that various features shown in different embodiments may be combined, and other changes and modifications may be made within the spirit and scope of the inventon as dened in the claims.

1 claim:

1. A transducer arrangement comprising a conduit, a first acoustic wave transmitting and receiving means and a second acoustic wave transmitting and receiving means displaced from each other along the conduit and located in the walls of said conduit for acoustic contact with a wave propagating medium adapted to flow through the conduit for transmitting waves to and receiving waves from each other, each of said means comprising a housing having a wall facing the -like wall of the other means, a pair of piezo-electric transducers on said wall inside said housing, an input lead connected to a first of said transducers and an output lead connected to the second of said pair of transducers, whereby the waves are adapted to travel between said first and second transmitting and receiving means in opposite directions through the wave propagating medium along nearly coincident paths.

2. A transducer arrangement according to claim 1 wherein each vof said transmitting and receiving means includes means for focusing the waves transmitted thereby on said wall of the other transmitting and receiving means.

3. A transducer arrangement according to claim 2 wherein the external face of said wall of each housing is spherically concave.

v4. A transducer arrangement according to claim 2 wherein the external face of said wall of each housing is cylindrically concave. f

5. A transducer arrangement according to claim 3 wherein said wall is circular, said first piezo-electric trans- Y ducer is a circular slab located at the center of said wall and said second piezo-electric transducer is ring-shaped and surrounds the circular slab.

References Cited in the tile of this patent UNITED STATES PATENTS 2,384,465 Harrison Sept. 11, 1945 2,477,246 Gillespie July 26, 1949 2,480,535 Alois et al. Aug. 30, 1949 2,645,727 Willard July 14, 1953 2,657,319 Smack- Oct. 19, 1953 2,669,121 Garman et al. Feb. 16,1954 2,708,366 Blocher etal. May-17, 1955 2,748,369 Smyth May 29, 1956 FOREIGN PATENTS 352,040 Great Britain June 29, 1931 603,644 Great Britain June 21, 1948 y 616,794 Great Britain Jan. 27, 1949 623,022 Great Britain May 11, 1949 

